Intro to Concrete18.0 & 19.0

Class notes

Objectives

• AM18.01 Describe what is in concrete and its characteristics

• AM18.02 Explain the procedure for mixing concrete.

• AM19.01 Explain how to prepare the site, how to pour, finish, and cure the

concrete project.• AM19.02 Demonstrate how to estimate the

amount of concrete needed for a job.

• I. What is in concrete and its characteristics?– A. Concrete is a mixture of Portland

cement, sand, gravel, and water.

– 1. Portland cement a dry powder made by burning limestone and clay, and then grinding and mixing to an even consistency.

» a. Packaged in bags containing 1 cubic foot and weighs 94 pounds.

» b. When mixed with water, it forms the “glue” to hold the aggregate together.

– 2. Fine Aggregate: sand and other small particle of stone.

» a. Washed sand: sand flushed with water to remove clay and silt (Dirt impurities)-fills in around course

– 3. Course aggregates are gravel or rocks over ¼" in diameter.-strength

– 4. Water should be clean.

• B. Most (about ¾) of all concrete used in construction is delivered by trucks to the construction site as ready-mixed concrete.

• C. Mortar a mixture of sand, portland cement, water and finishing lime.

1. Finishing Lime: a powder made by grinding and treating limestone.

• D. Although many people call concrete “cement”, cement is the powdered material and concrete is the mixture of ingredients.

• E. A cubic foot of concrete weighs from 150-160 pounds.

• II. Site preparation– A. Footing- part of the foundation

that distributes the weight of the house over a large area

– B. Form is a wood or metal structure that confines concrete to the desired shape until it hardens.

1. Trenches are dug in the ground for footings of buildings, and the soil walls are the forms.

2. Wood or metal can be used.

a. When wood is used, 2" x 4", 2" x 6", 2" x 8“

(1.) 2" size lumber is used because forms must not bend or warp and must be braced to prevent movement.

3. In warm weather, forms should stay in place 2-3 days and twice that long if weather is cold.

• III. Mixing concrete – A. The quality or strength of a mix

is determined by:• 1. Slump is the consistency or

wetness of the mixture.

• a. Stronger concrete requires less slump

b. If concrete has too much slump (is too wet), adding aggregates such as sand and gravel will correct problem.

• 2. Uniformity is thoroughly mixing all ingredients so that the “glue” which is wet cement will be around all sand and gravel particles (aggregates).

• 3. Workability refers to the ease with which concrete can be placed and shaped.

– B. Mixing Ratio is expresses as a three-digit number or ratioExample: 1-2-3.

1. The first number is parts of cement (1)2. Second number is parts of sand (2)(fine aggregate)3. Third number is parts of gravel (3)(large aggregate)4. Parts may be shovels full, pounds, cubic feet5. Less sand, gravel, and water are used for the same amount of

water to make concrete stronger.• C. Base ingredients- Hand mixing may be done in a wheelbarrow

or mixing box by using a shovel or hoe.• 1. Add sand and then cement and mix thoroughly.

• a. Then add and mix gravel.• b. Last thing to add is water and mix well.

• D. Machine mixing uses a concrete or mortar mixer. (Tub Mixing)• 1. First, add about 10% of water needed so that

ingredients will not stick to mixer. (Tub)• a. Then add gravel, then cement, then sand, (MIX) and then add

remainder of water needed.• b. Mix about two minutes after all ingredients are in the mixer.

(TUB)

• B. Finishing is bringing the surface of concrete to the proper grade and smoothness.

1. Screeding is using a straightedge such as a 2" x 4" board to level

concrete with top of forms.2. Bull float is used to make it easier to

finish.3. Texture - smoothness or roughness

of concrete surface.a. Wood float or stiff broom, will make a

gritty-non-slip surface b. For a very smooth surface, a

steel cement-finishing trowel should be used.

• C. When pouring a slab, must be sloped to allow water to run off- ¼ inch per foot of width

• D. Reinforcement is needed because concrete has low tensile strength and high compressive strength.– 1. Tensile strength- stretching

ability- low – 2. Compression- force to

crush-high – 3. Reinforcing helps prevent

cracking or breaking.– 4. Rebar (Clean steel rods) or

clean welded wire are used in concrete for reinforcement.

IV. Calculating concrete needed for a jobA. Concrete is sold in whole Cubic YardsB. Cubic yards Equation

1. Cu.yd.= Length(FT) x Width(FT) x Thickness(FT) 27 cubic ft/cubic yd

C. Problem-drive way1. Length= 50 ft.2. Width= 8 ft.3. Thickness= 6 inches

D. Substitute values into equation1. Cu. Yds= 50’(L)x 8’(W)x 6/12’ (T)

27 cu ft / cu yd2. Cu Yds. = 200’cu.ft. = 7.4 cu yds

27cu ft/cu yd3. BUY 10 % MORE THAN NEEDED

A. 7.4 X 1.10 = 8.14 CU YDS3. Must buy in whole Cu. Yds= 8.14 = 9.0 cu yds

MasonryI. Blocks

A. Standard concrete blocks

1. 8 x 8 x 16

2. 71/2 x 7 ½ x 15 ½

B. Web- Middle section

of block-

- large side up

C. Rows are called courses

D. Mortar joints – ½ in

Masonry

II. Lay outA. Set footingsB. Dry fit blocks

1. Middle of blocks2. 36 inches-outside edge to outside

edge 3. Diagonals4. Mark Story board5. Do course 2 & 36. Mark edge of

blocks

Masonry

III. Mortar- mixing

A. 15 min limit

B. Brick trowel & mortar board

C. Mixing

1. 2 scoops mortar

2. Add ½ scoop water-slowly

3. Mix with trowel

Masonry

Masonry